With the advancement of information communication technologies, various wireless communication technologies have recently been developed. Among the wireless communication technologies, a wireless local area network (WLAN) is a technology whereby Internet access is possible in a wireless fashion in homes or businesses or in a region providing a specific service by using a portable terminal such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc.
The IEEE 802.11n is a technical standard relatively recently introduced to overcome a limited data rate which has been considered as a drawback in the WLAN. The IEEE 802.11n is devised to increase network speed and reliability and to extend an operational distance of a wireless network. More specifically, the IEEE 802.11n supports a high throughput (HT), i.e., a data processing rate of up to above 540 Mbps, and is based on a multiple input and multiple output (MIMO) technique which uses multiple antennas in both a transmitter and a receiver to minimize a transmission error and to optimize a data rate.
With the widespread use of the WLAN and the diversification of applications using the WLAN, there is a recent demand for a new WLAN system to support a higher throughput than a data processing rate supported by the IEEE 802.11n. A next-generation WLAN system supporting a very high throughput (VHT) is a next version of the IEEE 802.11n WLAN system, and is one of IEEE 802.11 WLAN systems which have recently been proposed to support a data processing rate of above 1 Gbps in a MAC service access point (SAP).
The next-generation WLAN system supports the transmission of a Multi-User Multiple Input Multiple Output (MU-MIMO) scheme in which a plurality of non-AP STAs accesses a radio channel at the same time in order to efficiently use the radio channel. According to the MU-MIMO transmission scheme, an AP can transmit a frame to one or more MIMO-paired STAs at the same time.
The AP and the plurality of MU-MIMO paired STAs may have different capabilities. In this case, a supportable bandwidth, modulation coding scheme (MCS), forward error correction (FEC), etc., may vary depending on an STA type, usage, channel environment, etc.
In a WLAN system, an Access Point (AP) or an STA or both may obtain information about a channel to be used in transmitting a frame to a target reception AP or STA or both. This may be performed through a channel sounding procedure. That is, a process in which a transmitter requests channel information to be used for frame transmission and reception from a receiver and the receiver estimates the channel and feeds the channel information about the channel to the transmitter may be performed before the transmission and reception of a data frame. Meanwhile, the next-generation WLAN system may receive a greater amount of channel information from a target transmission AP or STA or both because a wider channel bandwidth and an MU-MIMO transmission scheme are adopted. In order to transmit more feedback information, the target transmission AP or STA or both have to access channels for a longer time.
An AP or an STA or both may do not normally receive necessary control information and data while performing a procedure for channel sounding. In this case, an STA or STAs or both that are intended to estimate channels may do not estimate the channels or do not transmit a feedback frame because they do not know whether channel sounding has been started. In this case, an AP or an STA or both that have started channel sounding starts channel sounding again from the beginning because they have not received the feedback frame. Accordingly, there are problems in that an STA or STAs or both that have already estimated channels consume power due to unnecessary operations and the channels are unnecessarily occupied. For this reason, there is a need to introduce a channel sounding method capable of solving the problems.